This invention relates generally to the exploitation of hydrocarbon-bearing formations having substantial electrical conductivity, such as tar sands and heavy oil deposits, by the application of electrical energy to heat the deposits. More specifically, the invention relates to the delivery of electrical power to a conductive formation at relatively low frequency or d.c., which power is applied between rows of elongated electrodes forming a waveguide structure bounding a particular volume of the formation, while at the same time the temperature of the electrodes is controlled.
Materials such as tar sands and heavy oil deposits are amenable to heat processing to produce gases and hydrocarbons. Generally the heat develops the porosity, permeability and/or mobility necessary for recovery. Some hydrocarbonaceous materials may be recovered upon pyrolysis or distillation, others simply upon heating to increase mobility.
Materials such as tar sands and heavy oil deposits are heterogeneous dielectrics. Such dielectric media exhibit very large values of conductivity, relative dielectric constant, and loss tangents at low temperature, but at high temperatures exhibit lower values for these parameters. Such behavior arises because in such media ionic conducting paths or layers are established in the moisture contained in the interstitial spaces in the porous, relatively low dielectric constant and loss tangent rock matrix. Upon heating, the moisture evaporates, which radically reduces the bulk conductivity, relative dielectric constant, and loss tangent to essentially that of the rock matrix.
It has been known to heat electrically relatively large volumes of hydrocarbonaceous formations in situ. Bridges and Taflove U.S. Pat. No. Re. 30,738 discloses a system and method for such in situ heat processing of hydrocarbonaceous earth formations wherein a plurality of elongated electrodes are inserted in formations and bound a particular volume of a formation of interest. As used therein, the term "bounding a particular formation" means that the volume is enclosed on at least two sides thereof. The enclosed sides are enclosed in an electrical sense with a row of discrete electrodes forming a particular side. Electrical excitation between rows of such electrodes established electrical fields in the volume. As disclosed in such patent, the frequency of the excitation was selected as a function of the bounded volume so as to establish a substantially nonradiating electric field which was confined substantially in the volume. The method and system of the reissue patent have particular application in the radio-frequency heating of moderately lossy dielectric formations at relatively high frequency. However, it is also useful in relatively lossy dielectric formations where relatively low frequency electrical power is utilized for heating largely by conduction. The present invention is directed toward the improvement of such method and system for such heating of relatively conductive formations at relatively low frequency and to the application of such system for heating with d.c.